US2922072A - Image reproduction device - Google Patents

Description

Jan. 19, 1960 F. K. COLLINS ETAL 2,922,072

7 IMAGE REPRODUCTION DEVICE Filed Dec. 5, 1957 INVENTORS FLOYD Kl COLL/IVS 0554/? A. DRAKE zmw ATTOR N EY IMAGE REPRODUCTION DEVICE Floyd K. Collins and Oscar A. Drake, Seneca Falls, N.Y., assignors, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, DeL, a corporation of Delaware Application December 5, 1957, Serial No. 700,848

1 Claim. 01. 315-16) tial applied to these electrodes is of a constant value since dynamic focusing using a varying potential is complicated and expensive to achieve. needed to focus the electron beam or beams in such tubes has heretofore been dependent upon the beam current. Since this current is being continuously modulated or varied during tube operation, the beam is not correctly focused over the major portions of the reproduced picture or display regardless of the arbitrary focusing voltage selected.

Other aspects of the beam forming problems which are interrelated with focusing concerns the actual shape of the beam spots and the haze which surrounds it when viewed at the impinging position on the screen. Very often, electrode voltages capable of producing the best focusing characteristic with a given tube structure also produces an undesirably distorted and hazy beam spot.

Accordingly, an object of the invention is the provision of substantially constant focusing for cathode ray tubes.

A further object is the provision of substantially constant focusing with the maintenance of a substantially undistorted electron beam spot having a minimum amount of haze.

A further object is to improve the fabrication of cathode ray tubes and the electrode gun mounts employed therein.

The foregoing objects are achieved in one aspect of the invention by the provision of an electrostatic focus type cathode ray tube employing a screen grid spaced from an anode having an aperture formed therein. Substantially constant beam focus with optimum beam spot quality is achieved by providing a specific ratio of the anode aperture to the spacing between the grid and anode when these electrodes have given prescribed voltages imposed thereon relative to one another.

For a better understanding of the invention, reference is made to the following description taken in conjunction with the accompanying drawings in which:

Fig. 1 is a partially sectioned plan view of a cathode ray tube;

Fig. 2 diagrammatically illustrates the electrode mount configurations and the beam shape for an electrode mount of the type adapted to be employed with the cathode ray tube shown in Fig. 1; and 1 Fig. 3 is a graphical representation of the manner in which the focusing voltage behaves as the cathode or beam current is varied.

Referring to the drawings, a cathode ray tube 11 is shown comprising an envelope 13 having an electron gun 15 disposedin the neck portion 17 thereof. The electron gun provides the source, control, acceleration and The voltage 2 focusingof the electron beam 19, which scans screen 21 under the influence of the magnetic fields provided by deflection coils 23.

The electron gun 15 comprises a cathode 25, control grid-27, screen grid 29 and a first anode 31 arranged substantially along the axis of the tube to provide the source, control, pre-focusing and acceleration of beam 19. An electrostatic focusing assembly comprising a first lens cup 33, lens ring 35 and second lens cup 37 is shown mounted upon common insulating support rods 39 with the other electrodes. If desired, the focusing electrodes may be mounted separately, and the'first anode 31 and first lens cup 33 may be fabricatedas one part, as shown, or as separate parts. During operation of tube '11,'the video signal is generally coupled to cathode 25 to provide a bias relative to grid 27. A voltage V of approximately 300 volts may: be imposed upon screen grid 29 while first anode 31, first lens cup 33 and second lens cup 37 may be coupled to voltage V of 16 kv. or over. Generally, lens ring 35 is operated at a voltage V, up to 300 volts. The lower voltages may be applied to their respective electrodes through lead-in base connections (not shown) to the tube while the high potentials are applied through internal connections (not shown) with the second anode conductive coating 41.

Electron beam 19 is focused in the manner shown in Fig. 2. The electrons emitted from cathode 25 proceed through the apertures in grids 27 and 29 to a crossover point 30. The electrostatic field between anode 31 and grid 29 forms the electrons into a beam which extends through the apertures in lens cups 33 and 37. The pri mary focusing is achieved by the electrostatic fields or lens existing in the vicinity of lens ring 35. Beam 19 is thereby focused to provide a small beam spot on screen 21.

It has been found that spacing S between the internal surface of the base 26 of cup-shaped grid 29 and the edge of tubular anode 31 disposed adjacent the open mouth portion 28 of grid 29 along with the diameter D of the aperture in anode 31 are instrumental in achieving constant focus of beam 19 for a wide range of beam current values with minimum distortion and haze. A ratio D/S greater than 1.3 has been found to produce the desired results. Referring to Fig. 3, it can be seen that a structure of the type described herein provides constant focus voltage E, for a wide range of beam currents I It has been found that as the spacing S becomes too small, e.g., below .050 inch, the curve loses its linearity,

at all values of beam currents.

The ratio of D/S which has been found satisfactory is between 1.3 and 6.0 when the voltage V on anode 31 is at least 10 times greater than the voltage V imposed on grid 29. Results have indicated that for anode voltages even below 6 k.v., the ratio still provides good focusing characteristics. I

Excellent results have been attained by using a spacing S of .145 inch with an anode aperture diameter D of .235 inch when V and V are 300 volts and 16 k.v. respectively. Varying within reasonable limits the length and width of anode 31 and the length and width of grid 29 has been found not to materially affect the'focusing characteristics of the tube. 1

Spot hazing may be improved somewhat by varying the aperture diameter W in lens cup 33 in accordance with the length L of the integral structure comprising anode 31 and lens cup 33.- Generally, it has been found that the aperture diameter should vary with the length L of the anode cup cylinder to provide minimum spot hazing. For instance, with a length L of .650 inch, aperture size W is found to be approximately .060 inch, i.e. L/ W=l0.8, whereas fora value L of .775 inch, a satisfactory aperture diameter W is .090 inch, i.e. L/W=8.6.

Although the electrodes described herein generally have cylindrical forms, it is to be understood that the invention is not restricted to such an electrode. For instance, grid 29 may be formed as an apertured disc, if desired. Also, anode 31 may "be formed as a straight cylinder and may be fabricated separately from the first lens cup.

Although several embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claim:

What is claimed is:

An image reproduction device electrode structure employing a plurality of axially aligned electrodes comprising, in order, a cathode providing an electron beam, a control grid, a'screen grid havingsa base portion and an open mouth portion, a tubular anode spaced from said base portion a distance S ranging from .050 to .200 inch formed with an aperture of diameter D which is smallerthan said open mouth portion, and an electrostatic beam focusing lens comprising a first lens cup integral with said anode having a re-entrant wall portionformed to provide an aperture of diameter W, the integral anode and first lens cup having an overall length L, a lens ring, and a second lens cup, the electrode structure providing substantially constant focusing of said beam when the ratio D/S ranges from'1.3 to 6.0, the ratio of L/ W rangesfrom 8.6.to- 10.8, and the voltage imposed upon said anode is greater than ten times the voltage imposed upon said screen grid.

References Cited in the file of this patent UNITED STATES PATENTS Knochel et al. Sept. 23, 1958

Images